Many modem aircraft are equipped with an airborne auxiliary power unit (“APU”) that provides electrical and pneumatic power to various parts of the aircraft. Typically, APUs include mounts, inlet and exhaust, and pneumatic (bleed) and fire containment systems, including systems for detecting and extinguishing fires. FIG. 1 shows a typical airborne APU 10 installed in the tailcone of an aircraft (shown in dashed outline 15). Some of the systems that support the APU 10 include an inlet system 20, a mount system 25, a pneumatic system 30, and an exhaust system 35. The APU exhaust system 35 may perform various functions, including evacuating APU exhaust gas from the aircraft 15, attenuating exhaust noise, and providing cooling airflow for the aircraft APU compartment when used in conjunction with an eductor system.
To attenuate exhaust noise, the exhaust system 35 may include a muffler section, such as muffler section 100, shown in FIG. 2, which includes an outer can 105 that houses a series of baffles 110 and an acoustic liner 115. Forward 120 and aft 125 end caps terminate the muffler section 100 and are typically attached to components of an eductor system (such as a bellmouth 130 or primary pipe) and the remainder of the exhaust piping 135.
A prior art example of acoustic liner 115 is shown in FIG. 3. The depicted acoustic liner 115 is fabricated from a FeCrAlY alloy, typically referred to as feltmetal. The fabrication of the acoustic liner 115 involves rolling a sheet of feltmetal into a cylinder and then forming a longitudinal weld joint 155. After the acoustic liner 115 is formed to the appropriate shape and welded, it is inserted into the muffler section 100, and attached thereto, typically by riveting 160 the acoustic liner 115 to the bellmouth 130, as shown in FIGS. 4 and 5.
APU exhaust systems having feltmetal acoustic liners typically have a service life of between about 7,000 and 10,000 aircraft hours. When compared to a typical aircraft service life of about 30,000 hours, the APU exhaust system 35 may need to be repaired or replaced several times during the life of the aircraft 15.
The short service life has been attributed to several different factors. For example, the longitudinal weld 155 may introduce a heat affected zone adjacent thereto. Analysis indicates that low cycle fatigue and inter-granular cracking may occur within the heat affected zones of the welds 155. Additionally, the welding process itself may introduce a second phase in the FeCrAlY mesh, which may weaken grain interfaces and contribute to material cracking. Moreover, the holes that are drilled in the feltmetal acoustic liner to accommodate the rivets 160 can introduce stress concentrations, which may in turn initiate fatigue cracking associated with thermal cycling and operational vibration.
Accordingly, there is a need to reduce the failure modes most commonly experienced by the acoustic liner by eliminating the longitudinal weld in the acoustic liner and also eliminating the annular pattern of rivets used for attachment of the acoustic liner. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.